Re: [PATCH v15 17/23] drm/shmem-helper: Add and use drm_gem_shmem_resv_assert_held() helper

[Dmitry Osipenko]

On 8/28/23 13:12, Boris Brezillon wrote:
> On Sun, 27 Aug 2023 20:54:43 +0300
> Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> wrote:
> 
>> In a preparation of adding drm-shmem memory shrinker, move all reservation
>> locking lockdep checks to use new drm_gem_shmem_resv_assert_held() that
>> will resolve spurious lockdep warning about wrong locking order vs
>> fs_reclam code paths during freeing of shmem GEM, where lockdep isn't
>> aware that it's impossible to have locking contention with the fs_reclam
>> at this special time.
>>
>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
>> ---
>>  drivers/gpu/drm/drm_gem_shmem_helper.c | 37 +++++++++++++++++---------
>>  1 file changed, 25 insertions(+), 12 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
>> index d96fee3d6166..ca5da976aafa 100644
>> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
>> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
>> @@ -128,6 +128,23 @@ struct drm_gem_shmem_object *drm_gem_shmem_create(struct drm_device *dev, size_t
>>  }
>>  EXPORT_SYMBOL_GPL(drm_gem_shmem_create);
>>  
>> +static void drm_gem_shmem_resv_assert_held(struct drm_gem_shmem_object *shmem)
>> +{
>> +	/*
>> +	 * Destroying the object is a special case.. drm_gem_shmem_free()
>> +	 * calls many things that WARN_ON if the obj lock is not held.  But
>> +	 * acquiring the obj lock in drm_gem_shmem_free() can cause a locking
>> +	 * order inversion between reservation_ww_class_mutex and fs_reclaim.
>> +	 *
>> +	 * This deadlock is not actually possible, because no one should
>> +	 * be already holding the lock when drm_gem_shmem_free() is called.
>> +	 * Unfortunately lockdep is not aware of this detail.  So when the
>> +	 * refcount drops to zero, we pretend it is already locked.
>> +	 */
>> +	if (kref_read(&shmem->base.refcount))
>> +		drm_gem_shmem_resv_assert_held(shmem);
>> +}
>> +
>>  /**
>>   * drm_gem_shmem_free - Free resources associated with a shmem GEM object
>>   * @shmem: shmem GEM object to free
>> @@ -142,8 +159,6 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
>>  	if (obj->import_attach) {
>>  		drm_prime_gem_destroy(obj, shmem->sgt);
>>  	} else if (!shmem->imported_sgt) {
>> -		dma_resv_lock(shmem->base.resv, NULL);
>> -
>>  		drm_WARN_ON(obj->dev, kref_read(&shmem->vmap_use_count));
>>  
>>  		if (shmem->sgt) {
>> @@ -156,8 +171,6 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
>>  			drm_gem_shmem_put_pages_locked(shmem);
> 
> AFAICT, drm_gem_shmem_put_pages_locked() is the only function that's
> called in the free path and would complain about resv-lock not being
> held. I think I'd feel more comfortable if we were adding a
> drm_gem_shmem_free_pages() function that did everything
> drm_gem_shmem_put_pages_locked() does except for the lock_held() check
> and the refcount dec, and have it called here (and in
> drm_gem_shmem_put_pages_locked()). This way we can keep using
> dma_resv_assert_held() instead of having our own variant.

It's not only drm_gem_shmem_free_pages(), but any drm-shmem function
that drivers may use in the GEM's freeing callback.

For example, panfrost_gem_free_object() may unpin shmem BO and then do
drm_gem_shmem_free().

-- 
Best regards,
Dmitry